Learn Hardcore Greek: Στο σινεμά κάθομαι δίπλα στη φίλη μου, αλλά ο διπλανός μου μιλάει δυνατά στο κινητό. - At the cinema I sit next to my friend, but the person next to me talks loudly on the phone.

In Greek the subject pronoun (like εγώ = I) is usually left out, because the verb ending already shows who the subject is.
The ending -ομαι in κάθομαι clearly marks 1st person singular (I sit / I am sitting), so εγώ is not needed unless you want to emphasize it (for example: Εγώ κάθομαι δίπλα στη φίλη μου, όχι αυτός.).

What kind of verb is κάθομαι and why does it end in -ομαι?

Κάθομαι is a middle / passive‑type verb; it always appears with endings like -ομαι, -εται, -όμαστε etc., even though its meaning here is active (I sit / I am sitting).
Its basic present‑tense forms are: κάθομαι, κάθεσαι, κάθεται, καθόμαστε, κάθεστε, κάθονται.
The simple (aorist) form you will meet later is κάτσω (for example θα κάτσω = I will sit).

What exactly does στο in Στο σινεμά mean?

Στο is the combination σε + το (in/to + the, neuter singular).
So στο σινεμά literally means in the cinema or at the cinema; depending on context it can also be understood as to the cinema.
In writing and speech, σε + το almost always contracts to στο.

Why does σινεμά look the same after στο? Shouldn’t the ending change?

Σινεμά is an indeclinable noun borrowed from French (cinéma).
That means its form does not change for case or number: το σινεμά, του σινεμά, στο σινεμά all use the same word σινεμά; only the article or preposition changes.

How is the phrase δίπλα στη φίλη μου built? What does στη come from?

The basic structure is δίπλα σε (next to) + a noun.
Here we have δίπλα σε τη φίλη μου (next to the friend of‑mine), and σε + τη contracts to στη, giving δίπλα στη φίλη μου.
So: δίπλα (next to) + σε (to) + τη (the, fem.) + φίλη (friend) + μου (my).

Why is it στη φίλη μου and not στην φίλη μου?

The full form is στην (σε + την), but in standard spelling the final -ν of την / στην is usually dropped before most consonants, including φ.
So στην φίλη becomes στη φίλη in normal writing.
You will still sometimes see or hear στην φίλη; keeping the -ν is not wrong, just less common in this context.

What is the exact role of μου in στη φίλη μου and ο διπλανός μου?

Μου after a noun is an unstressed possessive pronoun meaning my.
Greek usually puts these possessives after the noun: η φίλη μου = my friend, ο διπλανός μου = the person next to me / my neighbor (in the row).
Because μου is unstressed, the main stress stays on the noun (φίλη, διπλανός).

Why is φίλη used here and not φίλος?

Φίλη is the feminine form of friend, and φίλος is the masculine form.
Since the sentence talks about a female friend, it uses η φίλη μου.
For a male friend you would say ο φίλος μου.

What does ο διπλανός μου literally mean, and why is it masculine?

The adjective διπλανός means next‑door / next‑to / adjacent.
Used with the article by itself, ο διπλανός (μου) means the person sitting next to me (or in general, my neighbor in a row or a building).
It is masculine (ο διπλανός) because the speaker is imagining that neighbor as male; for a woman you would say η διπλανή μου.

Is there any difference between μιλάει and μιλά?

Both μιλάει and μιλά are correct present‑tense 3rd person forms of the verb μιλάω / μιλώ (to speak, to talk).
In everyday modern Greek μιλάει sounds a bit more informal/colloquial, while μιλά is slightly shorter and may feel a bit more formal or written, but both are very common.

Why does the sentence use μιλάει and not λέει?

Μιλάω means to speak / to be talking, often for a longer action or a conversation, and it combines naturally with στο τηλέφωνο / στο κινητό (on the phone).
Λέω focuses on saying specific words or sentences.
So μιλάει στο κινητό = he/she is talking on the phone, while λέει κάτι στο τηλέφωνο = he/she is saying something on the phone.

What does δυνατά mean here, and what kind of word is it?

Δυνατά is the adverb form of the adjective δυνατός (strong, loud, powerful).
In this context μιλάει δυνατά means he/she is speaking loudly.
You can intensify it as μιλάει πολύ δυνατά (is talking very loudly).

What exactly is κινητό in στο κινητό, and why do we use στο again?

Κινητό is short for κινητό τηλέφωνο (mobile / cell phone) and is a neuter noun: το κινητό.
After the preposition σε, neuter το again contracts: σε + το κινητό → στο κινητό = on the mobile phone / on the cell.
So μιλάει δυνατά στο κινητό literally means he/she is speaking loudly on the mobile phone.